Electronic converters for light sources, comprising e.g. at least one LED (Light Emitting Diode) or other solid-state lighting means, may offer a direct current output. Such current may be steady or vary in time, e.g. in order to adjust the brightness emitted by the light source (so-called dimming function).
FIG. 1 shows a possible lighting arrangement comprising an electronic converter 10 and a lighting module 20 including, e.g., at least one LED L.
For instance, FIG. 2 shows an example of a lighting module 20 comprising an LED chain, i.e. a plurality of LEDs connected in series. As an example, FIG. 2 shows four LEDs L1, L2, L3 and L4.
Electronic converter 10 may comprise a control circuit 102 and a power circuit 12 (e.g. an AC/DC or DC/DC switching power supply) which receives at an input, a supply signal (e.g. from the mains) and provides at an output, via a power output 106, a voltage V0. Such a voltage may be steady or vary in time. E.g., control circuit 102 may set, via a reference signal Vref of power circuit 12, the voltage provided at output 106 for feeding lighting module 20. Lighting module 20 may include a current regulator 22, connected in series with light sources L for limiting the current.
There are many types of electronic converters, such as for example “buck”, “boost”, “buck-boost”, “flyback” or “forward” converters.
FIG. 3 shows the circuit diagram of a flyback converter 12, wherein a lighting module 20 is connected to output 106 of converter 12.
A flyback converter comprises a transformer T with a primary winding T1 and a secondary winding T2, an electronic switch S, such as an n-channel MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), a rectification diode D and an output capacitor C.
Specifically, transformer T may be modelled as an inductor Lm connected in parallel with primary winding T1, which represents the magnetising inductance of transformer T, and an ideal transformer with a given ratio of the numbers of turns 1:n.
Converter 12 receives at input, via two input terminals 110, a voltage Vin and provides at output, via two output terminals 106, a regulated voltage Vo. Voltage Vin may also be obtained from an AC input current, for instance via a rectifier, e.g. a diode-bridge rectifier, and optionally via a filter capacitor.
The first input terminal (positive terminal) is connected to the first terminal of primary winding T1 of transformer T, and the second input terminal (negative terminal) represents a first ground GND1. On the other hand, the second terminal of primary winding T1 of transformer T is connected via switch S to ground GND1. Therefore, switch S may be used for activating selectively the current flow through primary winding T1 of transformer T.
In a flyback converter, secondary winding T2 of transformer T is connected via a diode D to an output capacitor C. Specifically, the first terminal of rectification winding T2 is connected to the anode of diode D, and the cathode of diode D is connected to a first output terminal (positive terminal). On the other hand, the second terminal is connected directly to the second output terminal (negative terminal), which represents a second ground GND2, which due to the insulating effect of transformer T is preferably different from ground GND1 and therefore is denoted with a different ground symbol.
Finally, capacitor C is connected in parallel with output 106.
As a consequence, when switch S is closed (first operating interval), primary winding T1 of transformer T is connected directly to input voltage Vin. This causes an increase of the magnetic flux in transformer T. Therefore, the voltage across secondary winding T2 is negative and diode D is reverse biased. In this condition, output capacitor C provides the energy required by lighting module 20.
On the other hand, when switch S is opened (second operating interval), the energy stored in transformer T is transferred as flyback current to the secondary side.
Typically, both operating intervals are repeated periodically. For example, a converter normally also comprises a driver circuit 112, controlling the switching of switch S as a function of a feedback signal. For example, such driver circuit 112 may be designed for repeating the operating intervals at a fixed frequency, wherein the energy transfer is controlled via a PWM signal, i.e. the durations of the first and of the second time intervals are variable, while the sum of the durations is constant. Such PWM driving and the control of the durations of the operating intervals are well known and that they can be implemented e.g. via a feedback of the output voltage via an error amplifier. For example, in the case of a voltage control, the duration of the first time interval is increased until the (average) output voltage corresponds to a desired value.
An electronic converter is often required to generate a plurality of voltages. For example, this may be useful for generating a supply for the control circuit 102 and/or for the driver circuit 112.
For example, FIGS. 4 and 5 show various solutions of multiple output supplies, having e.g. two outputs 106a and 106b, by resorting again e.g. to a flyback topology.
In the diagram of FIG. 4, transformer T is provided with two secondary windings T2a and T2b (a winding for each output 106a and 106b). Moreover, on the secondary side for each secondary winding T2a and T2b there is provided a rectification/filter network of a typical flyback converter, respectively including a diode Da/Db and a capacitor Ca/Cb. Therefore, a way to obtain multiple outputs consists in arranging several windings on the same transformer.
The solution according to FIG. 5, on the other hand, provides one secondary winding T2 and derives the auxiliary output(s) (e.g. output 106b) from main output 106a via an additional converter 12b. 
For example, such converter 12b may be a switching converter, such as e.g. a buck converter. This solution is efficient in energy consumption, but requires a number of additional components. Therefore, it is a costly solution and it is rather bulky, especially with reference to its use for generating an auxiliary low-power supply.
Generally speaking, converter 12b may also be implemented via a linear regulator. This solution is simple and may be implemented easily with a low number of components, but has the drawback of a high dissipation of the linear regulator, so that the possible application field is restricted to situations wherein the voltage difference and the current supplied at output are low.